1. Field of the invention
The present invention relates to methods of analysis of specimen gas for at least one impurity, by feeding specimen gas to a plasma and analyzing the plasma for the impurity, and also to apparatus for carrying out such a method. The invention is especially suitable for the analysis of a gas containing an impurity in a very small concentration, typically of the order of 10 nanograms per liter or less and even one nanogram per liter or less. The impurity may be a gaseous impurity or may take the form of small solid particles distributed in the gas.
2. Description of the Prior Art
There have been many prior proposals for the analysis of gas specimens using high temperature plasma. A general discussion is to be found in "Basic and Applied in ICP Spectroscopic Analysis", Haraguchi, Kodansha, pages 91-95, (1986) (in Japanese). This publication describes apparatus which combines plasma producing means and an optical spectrometer or a mass analyzer. Plasma producing means mentioned include a high frequency inductively coupled plasma (ICP) device and a microwave induction plasma (MIP) device. Details of the devices are not given.
JP-A-1-309300 (corresponding to U.S. Pat. No. 4933650) describes a microwave plasma generating apparatus used for analyzing a component present at low concentration in an aerosol derived from a liquid. A gas for supporting the plasma and a carrier gas for the sample containing the aerosol produced by means of a nebulizer are fed to the plasma zone through the outer tube and the inner tube of a double tube structure respectively. The gases fed in both the tubes of the plasma device are said to be helium, nitrogen, argon etc. The plasma is energized at the plasma zone by microwave power fed to it by a waveguide. The plasma causes ionization, so that the plasma generating device may be combined with a mass spectrometer or an optical emission spectrometer, for analysis of the desired component.
JP-A-2-110350 discloses a method of analyzing an impurity element contained in a highly pure gas using ICP. A gas sample to be analyzed is fed to the plasma zone in a low concentration in argon as a carrier gas via a center tube, and two tubes surrounding the center tube also supply argon to the plasma zone. The ions produced by the plasma are analyzed by optical spectroscopy or mass spectrometry. The gas sample is typically a chlorosilane.
The first two prior art disclosures mentioned above do not describe methods of analyzing a gas specimen as such. The first document describes the reduction of an element to be analyzed to form a volatile hydride, and the second describes the analysis of an aerosol in a carrier gas, the aerosol therefore being formed from a solution containing the element to be analyzed. The third prior art document describes the production of an argon plasma and the use of argon as a carrier gas to carry the gas specimen to the plasma.